About Arb Calculator

What the site offers

The arb calculator answers one question: if you split a stake across every outcome of a market at the prices you enter, what happens in each case? It works in three modes.

• 2-way: two outcomes, such as a tennis match or a two-way handicap.
• 3+ outcomes: three to twenty mutually exclusive outcomes, such as home, draw and away.
• Back/lay: a back bet balanced against a lay bet at an exchange, with commission, for a cash stake or a free bet whose stake is not returned.

Each result shows the rounded stakes to enter, the return for every outcome and the lowest net result after rounding. The site is free to use, needs no account and shows no advertising.

Who it is for

The calculator is for adults who already understand decimal odds and want to check the arithmetic before they decide anything, and for readers who want to understand why a set of prices does or does not cover every outcome. It is not a tipping service and it cannot tell you whether a bet will be accepted.

If you are under the legal gambling age where you live, this site is not for you. If gambling has stopped feeling manageable, read Responsible play first.

A result you can check

Every tool states its inputs, formula and supported rules. We use independent arithmetic checks and known boundary cases to test outputs. Rounding happens for display after the underlying calculation.

Multi-outcome formula

• Add 1 ÷ decimal odds for every outcome. Shown as a percentage, this sum is the book percentage.
• Each stake is the total stake × (1 ÷ that outcome's odds) ÷ that sum. Stakes are rounded down to cents and leftover cents go to the largest remainders.
• Each return is the rounded stake × its odds. The reported net result is the lowest return minus the total stake.

Back/lay formula

• Cash stake: lay stake = back stake × back odds ÷ (lay odds − commission).
• Free bet, stake not returned: lay stake = free bet stake × (back odds − 1) ÷ (lay odds − commission).
• The lay stake is rounded to cents. Liability = rounded lay stake × (lay odds − 1). Commission applies only to a winning lay bet and assumes no other bets in the same exchange market.

Education, not real-money play

Arb Calculator does not accept bets or deposits, choose an operator or predict an outcome. An example stake is a number used to explain a calculation.

Sources and corrections

The calculator uses reciprocal decimal odds to allocate a total stake. Back/lay mode balances the two settlement outcomes after the entered commission. Examples are hypothetical, with the input assumptions and rounding shown alongside each result.

The worked examples on the arb calculator come from these test cases, and every build checks the calculator against an independent exact-fraction calculation. To report an error, email contact@arbcalculator.co with the inputs you used.

How our content is produced

Every page starts from the calculator's written specification and formulas. Explanations are written to match what the tool actually does, including its rounding rules and the cases it rejects.

The worked-example tables are not typed by hand. They are generated by running the calculator's own code when the site is built, so the figures in the text and the figures in the tool cannot drift apart.

The site does not use live odds feeds, operator data or anything entered by visitors. The few external facts we rely on, such as where to find gambling support, link to their source.

Editorial principles

• Worst case first: the headline result is the lowest outcome after rounding, never the best one.
• Assumptions on the page: every result lists the assumptions it depends on, such as commission applying only to a winning lay bet.
• No predictions or tips: we explain arithmetic, not which side to back or where to bet.
• No paid placement: the site carries no advertising, affiliate links or sponsored content.
• Risk next to the math: practical limits such as refused stakes, changed odds and void rules are explained alongside the formulas.
• Plain language: terms like book percentage and liability are explained where they are used.

Review and corrections

Before any release, the calculation code has to pass its full test suite: the fixed test cases behind the worked examples, rounding and formatting checks, and hundreds of randomised cases compared against a separate exact-fraction implementation. If any check fails, the site is not published.

If you think a result is wrong, email contact@arbcalculator.co with the mode and the exact inputs you used. Confirmed errors are fixed in the code and the test suite, and the corrected page is republished with a new modified date in the sitemap.

Who runs Arb Calculator

Arb Calculator is a self-hosted website at arbcalculator.co. It does not take bets, hold money or act for any bookmaker or exchange.

We do not publish named authors or credentials on this site. Instead of asking you to trust a byline, we publish the formulas and test cases so you can check any result independently. Questions about the site or the people behind it can be sent through the contact page.